Physiologically labile linkers or modifiers are useful for a variety of process, including therapeutic drug delivery. The utility of the linker or modifier may be further enhanced if cleavage of the linker regenerates at least one of the original components in an unmodified state without any vestige of the linker or modifier.
Several strategies have been investigated in clinical and preclinical settings to reversibly link or modify a compound. Such reversible conjugates are used to lessen toxic effects and improve pharmacological properties of the compound. To be effective, the reversible conjugate must remain stable in the bloodstream, yet allow for release of the compound after interaction of the conjugate with the target cell. Further, the cleavage of the linker or modifier must be such that the compound be allowed to reach its biochemical target and to interact effectively with it. Often, the compound must be released in an unmodified state.
Examples of reversible conjugates include prodrugs, derivatives of drugs which remain inactive in their prototype form but are metabolized in the body to generate the active drugs, and carriers such as antibody-drug conjugates. The formation of reversible conjugates has been shown to be useful in the development of antitumor chemotherapeutic drugs and in nucleotide delivery.
Rozema et al. (U.S. Pat. No. 8,137,695) have shown reversible modification of polyamines using dimethylmaleic anhydrides which form pH sensitive maleamide linkages. Delivery of the modified polymer to cells and internalization results in cleavage of the maleamide linkage in the reduced pH environment of endosomes to regenerate polymer amines.
In addition to pH sensitive linkages, peptide-containing linkages have been developed that are activated by proteases in vivo. Rozema et al. (U.S. Pat. No. 8,426,554) provided a means to reversibly regulate membrane disruptive activity of membrane active polyamines using steric stabilizers or targeting groups conjugated to polymeric amino-containing sidechains via a dipeptide p-amidobenzyl-carbamate spacer (PABC). According to the published design, in presence of proteolytic enzymes, hydrolysis of the anilide bond triggers a 1,6-elimination cascade that results in generation of a unmodified polycationic polymer with restored membranolytic properties.
Application of the self-immolative PABC spacer in prodrug design was originally proposed by Carl et al. (1981). This strategy combines cleavage of covalent anilide bond with spontaneous release of the desired substrate. PABC spacers have been extensively studied for controlled drug release of a therapeutic agent, particularly for anticancer therapy (Dorywalska et al. 2015, Zhang et al. 2014, Florent et al 1998, Toki et al. 2002, Shamis et al. 2004, Amir et al. 2005, Amir et al. 2005, Gopin et al. 2006, Zhang et al. 2013, Zhang et al. 2013), However, there is concern that the aza-quinone methide, also known as a quinonimine methide (QIM), generated during PABC elimination, can be a source of toxicity due to its propensity to react with N, O, and S-nucleophiles (Reboud-Ravaux et al. 2009).

Certain peptide proteolyzable pro-drugs lacking the PABC spacer have been described (Zhong et al. 2013, Cho K Y et al. 2012). However, a limitation of the prior described peptide proteolyzable pro-drugs is that they do not liberate the primary amine constituent of the parent drug in a rate comparable to that of self-immolating PABC type analogues. Cleavage of the C-terminal amino acid residue by endopeptidases during proteolysis of the prodrug appears to be the rate limiting step (Masquelier et al. 1980, Schmid et al. 2007, Schmid et al. 2007, Elsadek et al. 2010, Trouet et al. 1982).